The present invention is directed to a method an apparatus for modulating light, and more particularly to an integrated modulator for modulating laser light that is effective over a wide wavelength range.
Optical communications systems are typically based on light that is generated continuously from a laser, where the light is modulated in an external modulator. The external modulator may be, for example, a Mach Zehnder modulator formed on lithium niobate substrate, having an input waveguide coupled to the output form the laser. For ease of fabrication, however, it is preferred to integrate the modulator on the same chip as the laser, in which case the modulator is formed from a semiconductor material.
One form of readily integrable modulator is the electro-absorption modulator. An electro-absorption modulator may be formed in the waveguide that receives the output light from the laser, although it may also be formed as a separate component. An electro-absorption modulator uses a semiconductor material whose band gap, under normal conditions, is larger than the energy of the photons emitted by the laser. Therefore, the light output from the laser is transmitted through the modulator. When the modulator material is subjected to an electric field, however, the band gap reduces due to an electro-optic effect. When the modulator waveguide material includes bulk semiconductor materials, band gap reduces due to the Franz-Keldysh effect. When the modulator includes one or more semiconductor quantum wells, the band gap reduces as a result of the Stark effect.
If the band gap reduces by a sufficient amount, the band gap may become equal to or less than the photon energy of the light output from the laser, in which case the laser light is absorbed in the modulator. Therefore, application of a modulating voltage to the electro-absorption modulator results in a corresponding modulation in the light emitted from the modulator. The electro-absorption modulator is often fabricated in the form of a reverse-biased diode. The electro-absorption modulator has advantages over a Mach-Zehnder modulator formed in semiconducting material because the insertion loss of the electro-absorption modulator is lower and the electro-absorption modulator is typically shorter than a Mach-Zehnder modulator by about a factor of ten. Furthermore, the drive voltage for an electro-absorption modulator is typically around two volts, while a Mach Zehnder formed in semiconducting material may require ten volts.
In considering the design of an electro-absorption modulator, it is advantageous that signal absorption in the modulator is minimal when the modulator is in the transmissive state and that signal absorption is close to 100% when the modulator is in the absorptive state. Therefore, the band gap of the unexcited modulator material is preferably significantly higher than the energy of the laser photons so that transmission in the transmissive state is as high as possible.
On the other hand, the band gap of the unexcited modulator should not be too much greater than the photon energy. The band gap varies approximately linearly with electric field applied across the modulator. Consequently, in order to maintain constant extinction ratio, an increasingly large voltage has to be applied to the electro-absorption modulator when the band gap of the unexcited modulator material is significantly larger than the laser photon energy. The requirement of large modulation voltage reduces the bandwidth of the modulator. If the applied voltage is not increased, then the extinction ratio of the modulator may be reduced. Large modulation voltages are not desirable, since the drive electronics become more complex and consume more power, and the modulator itself suffers from increased heating.
The task of the designer, therefore, is to select a material for the electro-absorption modulator whose band gap is not so small as to produce significant transmission-state losses, nor so large as to require a large drive voltage. It has been found that a satisfactory compromise in band gap energy is that the unexcited semiconductor material has a band gap that differs in energy from the laser output by approximately 0.033 eV. This difference between modulator band gap and the laser photon energy is referred to as detuning. For optical communications lasers operating at about 1550 nm, the detuning corresponds to a wavelength difference of approximately 65 nm.
There are difficulties, however, when the output wavelength of the laser is tunable. For example, distributed Bragg reflector (DBR) lasers are typically tunable over a range of about 10 nm, and grating coupled, sampled reflector (GCSR) lasers are tunable over several tens of nm. Tuning such lasers moves the laser out of the optimum regime for operating the electro-absorption modulator. If tuned sufficiently far, the laser may tune into a region where the modulator absorbs the output even without an applied electric field, thus reducing the extinction ratio. Furthermore, the laser may tune into a region where the applied voltage required for a particular level of attenuation is significantly higher than at other wavelengths. In general, it is more difficult to produce higher drive voltages at high modulation frequencies, and so tuning the laser may result in reduced modulation bandwidth for the new wavelength. Alternatively, if the voltage is kept constant, the absorption of the light in the modulator under applied voltage may be incomplete, thus reducing extinction ratio.
There is, therefore, a need to reduce or avoid the deleterious effects of tuning the laser away from optimum detuning with the electro-absorption modulator. Furthermore, it is desirable that the modulation voltage applied to the electro-absorption modulator to achieve a selected extinction ratio be constant, regardless of wavelength.
Generally, the present invention relates to an electro-absorption modulator that is tunable along with the laser. Tuning the electro-absorption modulator permits optimum detuning to be maintained, even though the laser is tuned over several tens of nm. One approach to tuning the electro-absorption modulator is to heat the electro-absorption modulator.
One particular embodiment of the invention is directed to a semiconductor laser device that has a substrate and a semiconductor laser positioned on the substrate. The semiconductor laser produces output light that is tunable over a tuning range between a first wavelength and a second wavelength. An electro-absorption modulator is disposed to modulate the light produced by the semiconductor laser. The operating temperature of the electro-absorption modulator is tunable so as to maintain constant detuning over at least a portion of the tuning range.
Another embodiment of the invention is directed to an optical communications system that includes an optical transmitter, and optical receiver and an optical communication link coupled between the optical transmitter and the optical receiver. The optical transmitter has at least one laser operable at a plurality of wavelengths. The laser includes a substrate and a semiconductor laser positioned on the substrate and producing output light tunable over a tuning range between a first wavelength and a second wavelength. The laser also includes an electro-absorption modulator disposed to modulate the output light produced by the semiconductor laser, an operating temperature of the electro-absorption modulator being tunable to maintain constant detuning over at least a portion of the tuning range.
Another embodiment of the invention is directed to a method of operating a semiconductor laser modulated by an electro-absorption modulator. The method includes tuning output light from the semiconductor laser to a desired wavelength between first and second wavelengths and directing the output light through an electro-absorption modulator. The method also includes adjusting an operating temperature of the electro-absorption modulator so as to achieve a preselected extinction level in the electro-absorption modulator.
Another embodiment of the invention is directed to a laser device that includes means for tuning output light from the semiconductor laser to a desired wavelength between first and second wavelengths and means for directing the output light through an electro-absorption modulator. The laser device also includes means for adjusting an operating temperature of the electro-absorption modulator so as to achieve a preselected extinction level in the electro-absorption modulator.
Another embodiment of the invention is directed to a modulator that has a waveguide positioned on a substrate, the waveguide passing through an electro-absorption modulator disposed on the substrate. A heater is disposed on the substrate proximate the electro-absorption modulator.
The above summary of the present invention is not intended to describe each illustrated embodiment or every implementation of the present invention. The figures and the detailed description which follow more particularly exemplify these embodiments.